[utilities] Replace "since" with "as".

As a conjunction, "since" can be either causal or temporal, unlike "as".

source/utilities.tex

@@ -687,7 +687,7 @@
 \expects
 \tcode{false} is \tcode{true}.
 \begin{note}
-Since this precondition cannot be satisfied, the behavior
+As this precondition cannot be satisfied, the behavior
 of calling \tcode{unreachable} is undefined.
 \end{note}
 
@@ -2715,7 +2715,7 @@
            make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});
 \end{codeblock}
 \begin{note}
-Since the type \tcode{T} cannot be deduced from the argument list,
+As the type \tcode{T} cannot be deduced from the argument list,
 it is necessary for it to be supplied as an explicit template argument.
 \end{note}
 \end{itemdescr}